High Speed Uplink Packet Access (HSUPA) is a 3G mobile telephony protocol providing high speed uplink transmission.
In the 3GPP Release 6 (HSUPA) standard there are two downlink channels, namely E-RGCH and E-HICH, used for controlling the transmission power of the UE and which are based on the use of a ternary alphabet. Those two channels transmit one among three different signals to the User Equipment (UE): UP, DOWN or HOLD for E-RGCH, ACK, NACK or DTX for the E-HICH. These signals are merely transmitted as +1, −1 or 0, and are received as +S, −S, or 0, possibly plus noise.
As known by the man skilled in the art, when the UE has to detect a received signal that could be +S, −S or 0, plus some noise N, and a decision has to be taken on what was transmitted, it is necessary to set two thresholds:                1) a first threshold between 0 and +S in order to detect a +1 or a 0        2) a second threshold between 0 and −S so as to detect a 0 or a −1Usually the two thresholds are symmetrical so the problem is really reduced to setting one threshold.        
Now, as shown in FIG. 1, the use of a ternary alphabet results in the existence of three gaussian distributions, referred to as 11, 12 and 13, requiring the need to determine two thresholds for minimizing the misdetection as well as false alarms.
Now, the problem is that in the case of the two channels E-RGCH and E-HICH in HSUPA, but not exclusively, one has unfortunately no information on the value of S, nor on the noise N, deriving in a distribution of the signal around the three values −1, 0 and +1.
Consequently, receiving a signal Y alone does not allow the discrimination between the three possibilities: just noise N, or +S+N or, at least −S+N.
Therefore, there is a need for a specific mechanism allowing the determination of an appropriate threshold to be used in the detection process of a channel transmitting symbols based on a ternary alphabet.
WO 2007/088469 discloses a method that includes measuring, during a measurement period, values corresponding to noise and interference signals received from a channel. The measurement period coincides with a period when no signal is transmitted on the channel.
US 2008/200138 discloses a method wherein a level of a first signal received via first channel is determined. Additionally, a moving average of a plurality of determined levels of the first signal is obtained, and the moving average is subtracted from the determined level. The subtraction result is then used for estimating a noise variance of a second signal received via a second channel.
ZHuan Ye et al: “PHY 28-1 Energy Detection Using Estimated Noise Variance for Spectrum Sensing in Cognitive Radio Networks” Wireless Communications and Networking Conference, IEEE, 31.3.2008, pages 711-716, XP031243716 ISBN 978-1-4244-1997-5, discloses the use of an estimated noise variance to calculate the threshold used in the spectrum sensing based on energy detection.